_Ink sac_, a sac containing a dark, sticky liquid which may be thrown out through the funnel into the water. The opening is near the inner opening of the funnel. _Observations._ 1. What is the shape of the squid? To what is this shape adapted? 2. Identify the head and the well-developed eyes. 3. How many arms or tentacles are there? How are they arranged with reference to the mouth? What do you find on the distal ends of the arms? How do the arms vary as to size? What does the position and arrangement of the arms suggest as to their function? 4. Identify the exhalent siphon. Where may water enter the mantle cavity? Recalling the action of the siphons in the clam, suggest a method by which a squid is propelled through the water. In what direction must it swim? 5. Split the mantle along the ventral surface and spread apart. Identify the long plume-like gills, the ink sac, and the inner opening of the exhalent siphon. How many gills do you find? _Suggested drawings._ a. The squid side view. b. The squid from the ventral side with the mantle split open, arrows to show direction of water. _Summary._ 1. In what ways does a squid show relationship to the clam and the snail? 2. What has a squid gained through the reduction of its exoskeleton? What has it lost? What changes were necessary in its structure to offset the loss of an exoskeleton? 4. A COMPARATIVE STUDY OF MOLLUSCA _Materials._ Specimens of as many different kinds of mollusks as possible, charts, books. _Observations._ 1. What is the symmetry?[3] [3] This and the following questions are to be answered for each specimen. The answers may be tabulated if desired. 2. Is the body segmented or unsegmented? 3. Are lateral appendages present or wanting? 4. Is an exoskeleton present or wanting? If present, is it univalve or bivalve; if absent, what other means of protection has been developed to take its place? 5. Is the animal fixed, or is it free to move? If fixed, in what way? If it moves, what is the method and organ of locomotion? 6. What are the organs of respiration? What is their character? 7. How is food obtained? 8. What senses are probably present? What sense organs are present? 9. What is the habitat? 10. In what ways if any does the animal show degeneration? _Summary._ 1. What characters are common to all mollusks? 2. What is the principal means of protection among mollusks? 3. Name three causes of degeneration among mollusks. 5. MOLLUSCA: REVIEW AND LIBRARY EXERCISE _Characteristics._ 1. What are the general characteristics of mollusks? 2. Name the principal classes and give the characteristics of each. _Morphology._ 3. What is peculiar about the structure of a clam's heart? What is its position? Contrast with the heart of a crayfish.